Method for producing electrode for electric double layer capacitor

ABSTRACT

A method for producing an electrode for electric double layer capacitors is disclosed which comprises: a step of mixing a particulate elastomer and a carbonaceous material with each other in a powdery form, thereby obtaining a powdery mixture; and a step of dry-forming the resultant powdery mixture, thereby forming an electrode layer. With this method, an electrode which enables to form a high-capacity electric double layer capacitor can be produced by simplified steps with high productivity.

TECHNICAL FIELD

The present invention relates to a method for producing an electrode foran electric double layer capacitor.

BACKGROUND ART

About electric double layer capacitors using an electric double layerformed at interface of a polarizable electrode and an electrolytesurface, the demand thereof as memory backup electric power sources hasbeen rapidly expanding in recent years. Moreover, attention has beenpaid to the application thereof to articles for which a high capacity isrequired, such as a power source for a fuel cell mounted vehicle.

An electrode for an electric double layer capacitor has a structureobtained by molding an electrode-forming composition comprising a carbonmaterial as an active material, a binder and an optionalelectroconductivity additive to prepare an electrode layer, and thenlaminating the electrode layer on a metal foil, metal mesh or the likeas a current collector.

As the method for molding the electrode layer, press-molding is known.For example, Japanese Patent Application Laid-Open (JP-A-) Nos.63-107011 and 2-235320 each suggest a method of press-molding anelectrode-forming composition comprising fine carbon particles, afluorine-containing polymer such as polytetrafluoroethylene (PTFE), anda liquid lubricant to form an electrode layer. JP-A No. 9-306798 alsosuggests a method of integrating a kneaded product of activated carbonand PTFE as a binder with a current collector electrode made of metaland press-molding the integrated product to form an electrode.

However, in the case of using PTFE as a binder, it is necessary topre-knead the electrode-forming composition to turn PTFE into a fibrousform and it is also necessary to remove a kneading auxiliary agent addedat the time of the pre-kneading. In such a way, a problem that theprocess becomes complicated is caused. When PTFE is pre-kneaded, regionsmade into a fibrous form and regions not made into a fibrous form aregenerated; therefore, when an electrode layer in a thin film form isformed, the surface of the resultant easily becomes uneven. Thus, thestrength of the electrode may be insufficient or the performance of theelectric double layer capacitor to be obtained may not be sufficientlygood.

As a method using a binder other than PTFE, suggested is also a methodof molding activated carbon having a specific particle diameter andplastic powder having a specific particle diameter into a plate form ata temperature near to the melting point of the plastic (JP-A No.4-22062). Suggested is also a method of mixing activated carbon, anelectroconductive material, and binder powder made of a thermoplasticresin or B-stage thermosetting resin in a powdery state to obtain mixedpowder, and press-molding the mixed powder to form an electrode (JP-ANo. 63-151010). However, according to these methods, the resultantelectrode is insufficient in flexibility. Thus, when the electrode iswounded and then put into a container, its electrode layer may becracked or may fall away from the current corrector. The performance ofthe electric double layer capacitor to be obtained is also insufficient.

As a method using an elastomer as a binder, JP-A No. 62-16506 suggests amethod of mixing a latex and activated carbon, dehydrating the mixture,grinding and granulating the resultant aggregate, and then press-moldingthe granulated product. Furthermore, JP-A No. 8-250380 suggests a methodof press-molding a mixture obtained by mixing a solution of styrenebutadiene rubber or acrylonitrile butadiene rubber in xylene withactivated carbon and then drying the resultant. However, the methodsdescribed in these documents have complicated steps, and the performanceof the electric double layer capacitor to be obtained is alsoinsufficient.

DISCLOSURE OF THE INVENTION

In light of problems as described above in the prior art, an object ofthe present invention is to provide a method for producing an electrodefor giving an electric double layer capacitor having a high capacity,with a good productivity, through simplified steps.

The present inventors have made eager investigations to solve theabove-mentioned problems. As a result, the present inventors have foundout that: in a method of mixing a latex with activated carbon, pores inthe activated carbon are embedded with an emulsifier and so on in thelatex, and in the method of using plastic powder as a binder or in themethod of mixing a solution of styrene butadiene rubber or acrylonitrilebutadiene rubber in xylene with activated carbon, the surface of theactivated carbon is covered with the binder; therefore, the capacity ofthe electric double layer capacitor to be obtained may lower.

Furthermore, it has been found out that an electrode for an electricdouble layer capacitor can be effectively formed by mixing a particulateelastomer and a carbonaceous material in a powdery form and thendry-forming the powdery mixture obtained by the mixing, and an electricdouble layer capacitor wherein the electrode is used exhibits a highelectrostatic capacity. On the basis of these findings, the presentinvention has been made.

The present inventors have made the following invention based on thesefindings.

Thus, according to a first aspect of the present invention, provided isa method for producing an electrode for an electric double layercapacitor, comprising a step of mixing a particulate elastomer and acarbonaceous material with each other in a powdery form, therebyobtaining a powdery mixture, and a step of dry-forming the resultantpowdery mixture, thereby forming an electrode layer.

The particulate elastomer is preferably an elastomer having acrosslinked structure.

The carbonaceous material preferably comprises activated carbon as anactive material.

Preferably, the carbonaceous material further comprises anelectroconductivity additive.

The above-mentioned method for producing an electrode for an electricdouble layer capacitor preferably comprises a step of causing theelectroconductivity additive to adhere onto the surface of the activematerial by mechanochemical treatment.

The powdery mixture is preferably a mixture obtained by fluidized bedgranulation or fluidized bed multifunction mode granulation.

The particle diameter of the powdery mixture is preferably from 0.1 to1000 μm.

The dry-forming is preferably press-molding.

The press-molding is preferably performed inside a mold wherein acurrent collector is set.

Preferably, the powdery mixture comprises, in 100 parts by weightthereof, 0.1 to 50 parts by weight of the particulate elastomer and 50to 99.9 parts by weight of the carbonaceous material.

According to a second aspect of the present invention, provided is anelectrode for an electric double layer capacitor which is obtained bythe above-mentioned production method.

According to a third aspect of the present invention, provided is anelectric double layer capacitor comprising the above-mentionedelectrode.

BEST MODES FOR CARRYING OUT THE INVENTION

The following will describe components which constitute the electrode ofthe present invention for an electric double layer capacitor, and amethod for producing the electrode.

(1) Components Which Constitute the Electrode

The electrode layer of the electrode of the invention for an electricdouble layer capacitor comprises a particulate elastomer which is abinder, and a carbonaceous material.

<Particulate Elastomer>

In the invention, the particulate elastomer has a function as a binder.The particulate elastomer is used as a binder and is mixed with acarbonaceous material in a powdery form, thereby making even dispersionthereof possible. The particulate elastomer of the invention may be anelastomer made into a particle form by communition or the like, and ispreferably a polymer made into a particle form through a chemicalcrosslinked structure. The use of the polymer having the crosslinkedstructure makes it possible to keep the particle shape thereof stable.Specifically, a conjugated diene or a polyfunctional ethylenicallyunsaturated monomer can be made into a polymer having a crosslinkedstructure by homopolymerization or copolymerization thereof. Examples ofthe conjugated diene include butadiene and isoprene. Examples of thepolyfunctional ethylenically unsaturated monomer include dimethacrylicacid esters such as ethylene glycol dimethacrylate and diethylene glycoldimethacrylate; trimethacrylic acid esters such as trimethylolpropanetrimethacrylate; and divinyl compounds such as divinylbenzene.

These conjugated dienes or polyfunctional ethylenically unsaturatedmonomers may be copolymerized with a monofunctionalradical-copolymerizable monomer. Examples of the monofunctionalradical-copolymerizable monomer include acrylic acid esters such asbutyl acrylate and 2-ethylhexyl acrylate; methacrylic acid esters suchas butyl methacrylate and 2-ethylhexyl methacrylate; aromatic vinylcompounds such as styrene; ethylenically α,β-unsaturated nitrilecompounds such as acrylonitrile and methacrylonitrile; and ethylenicallyunsaturated carboxyl acids such as acrylic acid, methacrylic acid, anditaconic acid.

A preferred example of the polymer used as the particulate elastomer inthe invention is a copolymer made from a polyfunctional ethylenicallyunsaturated monomer and an acrylic acid ester. Specific examples thereofinclude 2-ethylhexyl acrylate/methacrylic acid/acrylonitrile/ethyleneglycol dimethacrylate copolymer, 2-ethylhexyl acrylate/methacrylicacid/methacrylonitrile/diethylene glycol dimethacrylate copolymer, butylacrylate/acrylonitrile/diethylene glycol dimethacrylate copolymer, andbutyl acrylate/acrylic acid/trimethylolpropane trimethacrylatecopolymer. Polybutadiene, polyisoprene or styrene/butadiene copolymerwhich may be modified with carboxyl group can also be preferably used.When these particulate elastomers, which have a crosslinked structure,are used as a binder, the ratio of the activated carbon surface or porescovered with the binder becomes small so that a rise in the internalresistance or a decline in the electrostatic capacity can be favorablyrestrained.

The particle diameter of the particulate elastomer is usually from0.0001 to 100 μm, preferably from 0.001 to 10 μm, and more preferablyfrom 0.01 to 1 μm. When the particle diameter is within this range, thehandling thereof is easy when it is mixed with the carbonaceous materialand further an excellent binding force is exhibited by use of a smallamount thereof. The particle diameter means the number-average particlediameter obtained by measuring the diameters of 100 polymer particlesselected at random in transmission electron microscopic photographsthereof and then calculating the arithmetic average of the diameters.

The glass transition temperature (Tg) of the particulate elastomer isusually from −60 to 20° C., preferably from −40 to 0° C. If the Tg istoo high, the binding force lowers. If the Tg is too low, the surface ofthe active material may be covered with the particulate elastomer toincrease the internal resistance.

In the invention, the amount of the used particulate elastomer isusually from 0.1 to 50 parts by weight, preferably from 1 to 20 parts byweight, and more preferably from 2 to 10 parts by weight for 100 partsby weight of the powdery mixture in order to obtain an electrode forgiving an electric double layer capacitor having a high capacity.

<Carbonaceous Material>

The carbonaceous material used in the invention comprises an “activematerial” made of a carbonaceous substance, and if necessary, thematerial comprises an “electroconductivity additive”.

Examples of the active material used in the invention include activatedcarbon, polyacene, and graphite or the like. There is used powder whosespecific surface area is usually 30 m²/g or more, preferably from 500 to5000 m²/g, and more preferably from 1000 to 3000 m²/g. There can also beused, as the active material, non-porous carbon having carbonmicrocrystal similar to graphite, the interlayer distance of the carbonmicrocrystal being enlarged, described in JP-A No. 11-317333 and JP-ANo. 2002-25867 or the like. The active material is preferably activatedcarbon. Specifically, activated carbon of a phenol type, a rayon type, apitch type, a coconut husk type or the like can be used. The particlediameter of the active material is preferably from 0.1 to 100 μm, morepreferably from 1 to 20 μm since the electrode for an electric doublelayer capacitor can be made thin and the electrostatic capacity thereofcan also be made high.

Examples of the carbonaceous material used as the electroconductivityadditive include carbon blacks such as furnace black, acetylene black,KETJENBLACK. The electroconductivity additive is used in the form of amixture with the active material. The particle diameter of theelectroconductivity additive is preferably from 0.1 to 100 μm. The useof the electroconductivity additive together causes a furtherimprovement in electrical contact of the active material particles tolower the internal resistance of the electric double layer capacitor tobe obtained and further make the electrostatic capacity thereof high.

When the electroconductivity additive is used in the state that theagent is caused to adhere onto the surface of the active material,respective particles thereof can be evenly dispersed. Thus, the case ispreferred. An example of the method for causing the electroconductivityadditive to adhere onto the surface of the active material ismechanochemical treatment of mixing the active material with theelectroconductivity additive while mechanical external force such ascompressive force or shearing force is applied thereto. As the devicefor conducting the mechanochemical treatment, a mechano-mill, ahybridizer, a mechano-fusion or the like can be used.

In the invention, the amount of the used carbonaceous material (theactive material and the electroconductivity additive) is usually from 50to 99.9 parts by weight, preferably from 70 to 98 parts by weight, andmore preferably from 80 to 96 parts by weight for 100 parts by weight ofthe powdery mixture in order to obtain an electrode for giving anelectric double layer capacitor having a high capacity. About the blendratio between the active material and the electroconductivity additive,the amount of the electroconductivity additive is from 0.1 to 20 partsby weight, preferably from 2 to 10 parts by weight for 100 parts byweight of the active material.

(2) Method for Producing an Electrode

<Preparation of a Powdery Mixture>

In the method of the present invention for producing an electrode for anelectric double layer capacitor, the above-mentioned particulateelastomer and carbonaceous material are first mixed with each other in apowdery form, thereby obtaining a powdery mixture.

In the invention, the wording “being mixed in a powdery form” means thatthe particulate elastomer and the carbonaceous material are mixed witheach other in the state that they are each kept in a granular form. Theymay contain water, a solvent or the like as long as each of them can bekept in a granular form. At the time of the mixing, the concentration ofsolid contents is usually 50% or more by weight, preferably 60% or moreby weight, and more preferably 70% or more by weight. When the solidcontent concentration is within this range, aggregation or the like ofthe particulate elastomer and the carbonaceous material is not caused sothat the granular form can be kept. When the resultant powdery mixturecontains water, a solvent or the like, the mixture is dried if necessaryand subjected to dry forming.

The mixer used for the mixing is not particularly limited if the mixeris capable of mixing the particulate elastomer and the carbonaceousmaterial in a powdery form. Specifically, a Henschel mixer, an Omnimixer or the like is preferably used. The mixing time is usually fromseveral seconds to about 1 hour, preferably from 1 to 5 minutes. Themixing temperature is not particularly limited, either, and is usuallyroom temperature.

The powdery mixture may be obtained by a granulating method such astumbling granulation, agitation granulation, fluidized bed granulationor fluidized bed multifunction mode granulation.

The tumbling granulation, agitation granulation, fluidized bedgranulation and fluidized bed multifunction mode granulation are methodsof spraying the particulate elastomer onto the carbonaceous materialwhich are forcibly caused to flow, thereby performing granulation. Inthese methods, the manners for causing the carbonaceous material to floware different from each other, and in the tumbling granulation, thecarbonaceous material and optional other components are tumbled inside arotary vessel such as a rotary drum or a rotary pan. In the agitationgranulation, a mixer such as a Henschel mixer is used to give flowingmotion forcibly to the powder of the carbonaceous material with stirringfans or the like which are set in its vessel. The fluidized bedgranulation is a method of keeping the powder of the carbonaceousmaterial in the state that the powder is floated and suspended in airflow blown up from the bottom. The fluidized bed multifunction modegranulation is a method of combining rolling or stirring action with thefluidized bed granulation. The temperature of the fluidized bedcontaining the carbonaceous material is usually from room temperature to100° C., and the particulate elastomer is sprayed usually at 50 to 250°C. Of the above-mentioned granulating methods, the fluidized bedgranulation and the fluidized bed multifunction mode granulation arepreferred since a powdery mixture having a small particle diameter iseasily obtained and the particle diameter is easily controlled.

The particulate elastomer used in the mixing may be dried elastomerparticles. Preferably, the carbonaceous material is charged into amixer, and a latex-form particulate elastomer, which is dispersed inwater, is sprayed and added thereto. According to the spray andaddition, water and the particulate elastomer are evenly adsorbed on thecarbonaceous material and further the carbonaceous material and theparticulate elastomer are kept in a powder form.

The particle diameter of the resultant powdery mixture is usually from0.1 to 1000 μm, preferably from 1 to 500 μm, and more preferably from 5to 100 μm. The particle diameter means the number-average particlediameter obtained by measuring the diameters of 100 particles of thepowdery mixture which are selected at random in transmission electronmicroscopic photographs thereof and then calculating the arithmeticaverage of the diameters. When the particle diameter is within thisrange, an electrode having a smooth surface and an even density can beobtained.

<Formation of an Electrode Layer>

The powdery mixture obtained as descried above is dry-formed into anelectrode layer. The dry-forming in the invention is a conceptionagainst what is called “wet-molding”, such as coating or spray. Examplesof such a method include press-molding, powder molding, rolling, andextrusion molding. Of these, the press-molding is preferred. In thedry-forming, the powdery mixture may be used in the state that themixture contains water, a solvent or the like. Water or a solvent may befurther added thereto as a molding auxiliary. At the time of themolding, the concentration of solid contents is usually 50% or more byweight, preferably 60% or more by weight, and more preferably 70% ormore by weight. The water or solvent can be removed by heating,pressure-reduction or the like when or after the electrode layer isformed.

In the press-molding, the powdery mixture is formed into an electrodelayer shape by a leaf type press, a roll type press or the like. For thepress-molding, preferred is a method of using a mold to form anelectrode layer inside the mold. According to this method, a series ofsteps of supplying the powdery mixture into the mold, press-molding themixture, and taking out the formed electrode layer can be madeautomatic. Thus, unmanned continuous production can be attained.Moreover, electrodes having different sizes and shapes can be producedonly by exchanging molds, and further the electrodes can be produced bymeans of small-sized molding equipment. Thus, this method is suitablefor the production of various kinds of electrodes. The temperature onpressing is varied in accordance with the glass transition temperatureand the particle diameter of the particulate elastomer, and others, andit is advisable to select the temperature within the range from roomtemperature to the decomposition temperature of the used particulateelastomer. The temperature is preferably a temperature 10 to 30° C.higher than the glass transition temperature (Tg). The pressure, whichdepends on the temperature, is not particularly limited as long as adesired electrode density can be obtained.

The thickness of the formed electrode layer is preferably from 50 to1000 μm, and the density of the electrode layer is preferably 0.5 g/cm³or more. They are decided dependently on the relationship thereof withan internal resistance desired in accordance with use purpose. If theinternal resistance is small, the density and the thickness of theelectrode layer can be made large. As a result, the energy density canbe made high. However, if the density of the electrode layer is made toohigh, the permeability of an electrolytic solution thereintodeteriorates. Thus, the density is preferably from 0.6 to 0.9 g/cm³.

<Production of an Electrode>

The electrode layer formed as described above is laminated onto acurrent collector, thereby obtaining an electrode. The current collectoris not particularly limited if the collector is made of anelectroconductive material. Preferred is a metal material such as iron,copper, aluminum, nickel or stainless steel. The metal material may bein the form of a sheet (metal foil), a film or a net. A carbon fiberwoven, a carbon mat, an electroconductive rubber sheet, and laminatesthereof can also be used as the current collector. Of these, the metalfoil is preferred. Aluminum foil is particularly preferred. Thethickness of the metal foil is preferably from 5 to 100 μm, inparticular preferably from 10 to 50 μm.

As the current collector, a current collector having surfaces on whichelectroconductive paste layers are formed may be used. Theelectroconductive adhesive agent is an agent having at least anelectroconductivity additive and a binder, and the agent can be producedby kneading the electroconductivity additive, the binder, and anoptional dispersing agent in water or an organic solvent. The resultantelectroconductive adhesive agent is applied onto the current collector,and dried to form a layer of the electroconductive adhesive agent. Theelectroconductive adhesive agent improves binding force between theelectrode layer and the current collector and further contributes toreduce the internal resistance.

The electroconductivity additive used in the electroconductive adhesiveagent may be any one of the electroconductivity additives exemplified inthe description of the above-mentioned electrode components. The binderthat can be used may be an elastomer, and is preferably theabove-mentioned particulate elastomer. The dispersing agent that can beused may be a cellulose such as carboxymethylcellulose, polyvinylalcohol, polyvinyl methyl ether, polyacrylic acid and its salt, oxidizedstarch, phosphorylated starch, casein, any one of various modifiedstarches, or the like. About the amount of each of the used components,the amount of the binder and that of the dispersing agent are preferablyfrom 5 to 20 parts by weight and from 1 to 5 parts by weight,respectively, for 100 parts by weight of the electroconductivityadditive, the amounts being amounts in terms of dry weight. If theamount of the used binder is too small, the adhesion between theelectrode layer and the current collector may become insufficient. Onthe other hand, if the amount of the used binder is too large, thedispersion of the electroconductivity additive becomes insufficient sothat the internal resistance may get large. If the amount of the useddispersing agent is too small, the dispersion of the electroconductivityadditive may become insufficient. On the other hand, if the amount ofthe used dispersing agent is too large, the electroconductivity additiveis covered with the dispersing agent so that the internal resistance mayget large.

The kneader used for the kneading is preferably a kneader capable ofapplying shearing force in order to make the dispersion of theelectroconductivity additive even. Specifically, there can be used aball mill, a sand mill, a pigment disperser, a crusher, an ultrasonicdisperser, a homogenizer, a planetary mixer, or the like.

The method for applying the electroconductive adhesive agent onto thecurrent collector is not particularly limited. The agent is applied by,for example, a doctor blade method, a dipping method, a reverse rollmethod, a direct roll method, a gravure method, an extrusion method,brush coating or the like. The coating amount thereof is notparticularly limited, and is adjusted in such a manner that thethickness of the electroconductive layer formed after the adhesive agentis dried will be usually from 0.5 to 10 μm, preferably from 2 to 7 μm.

The method for laminating the electrode layer onto the current collectorto obtain an electrode is not particularly limited. Examples thereofinclude a method of sticking a metal foil as the current collector ontothe electrode layer formed by press-molding, and a method of vapordepositing a metal into a film onto the electrode layer. When thepress-molding of the electrode layer is performed inside a mold, theabove-mentioned powdery mixture is supplied into the mold where thecurrent collector is set and then the mixture is subjected to presserforming, whereby the formation of the electrode layer is attained at thesame time when the electrode layer can be laminated onto the currentcollector. Thus, the process can be made simple.

When a sheet-form electrode layer is continuously formed by extrusionmolding or rolling, a roll-type metal rolled foil coil can be used asthe current collector and the metal foil can be continuously drawn outfrom the roll and be continuously laminated onto the electrode layer.The resultant sheet-form electrode may be further pressed to make theelectrode density thereof high.

(3) Electric Double Layer Capacitor

The electric double layer capacitor of the present invention is anelectric double layer capacitor having an electrode obtained by theabove-mentioned production method. The electric double layer capacitorcan be produced in accordance with an ordinary method using a pluralityof the above-mentioned electrodes, an electrolytic solution, and partssuch as a separator. Specifically, for example, the electrodes arestacked in the state that a separator is interposed therebetween, andthis is wounded or folded into a capacitor form and then put into acontainer. An electrolytic solution is poured into the container and thecontainer is sealed up. In this way, the electric double layer capacitorcan be produced.

The electrolytic solution used in the production of the electric doublelayer capacitor of the invention is not particularly limited, and ispreferably a non-aqueous electrolytic solution wherein an electrolyte isdissolved in an organic solvent.

The electrolyte may be any electrolyte known in the prior art, andexamples thereof include tetraethylammonium tetrafluoroborate,triethylmonomethylammonium tetrafluoroborate, and tetraethylammoniumhexafluorophosphate.

The solvent (electrolyte solvent) wherein these electrolytes aredissolved is not particularly limited if the solvent is a solvent whichis generally used as an electrolyte solvent. Specific examples thereofinclude carbonates such as propylene carbonate, ethylene carbonate, andbutylene carbonate; lactones such as γ-butyrolactone; sulfolanes; andnitrites such as acetonitrile. These may be used alone or in the form ofa mixed solvent composed of two or more thereof. Of these, carbonatesare preferred. The concentration of the electrolytic solution is usuallyfrom 0.5 mol/L or more, preferably 0.8 mol/L or more.

The separator that can be used may be a known separator, such as amicro-porous membrane or nonwoven cloth made of a polyolefin such aspolyethylene or polypropylene, or a porous film made mainly of pulp,which is generally called electrolytic capacitor paper. Instead of theseparator, a solid electrolyte or a gel electrolyte may be used.

EXAMPLES

The present invention will be more specifically described by way of thefollowing working examples and comparative examples. However, theinvention is not limited to these working examples. In the examples,“part(s)” and “%” are part(s) by weight and % by weight, respectively,unless otherwise specified. The particle diameter of polymers and thepowdery mixtures in the examples is the number-average particle diameterobtained by measuring the diameters of 100 particles thereof selected atrandom in transmission electron microscopic photographs thereof and thencalculating the arithmetic average of the diameters. The glasstransition temperature (Tg) of the polymers is a value measured with adifferential scanning calorimeter (DSC) at a temperature-elevation rateof 10° C./minute.

<Production of Electrode Layers and Electric Double Layer Capacitors>

Example 1

(Formation of an Electrode Layer)

While 170 parts of activated carbon (particle diameter: 8 μm, andspecific surface area: 2000 m²/g) were stirred with a Henschel mixer,thereto were sprayed and added 20 parts of an aqueous 40% dispersion ofcarboxyl-modified styrene/butadiene copolymer particles (Tg: −5° C., andparticle diameter: 12 μm) having a crosslinked structure over 10minutes. Next, 20 parts of acetylene black were added thereto over 10minutes, and the components were mixed to obtain a powdery mixturehaving a particle diameter of 163 μm.

Into a mold, 4 cm×6 cm, was supplied 4.5 g of the resultant powderymixture, and then the mixture was pressed at a pressing pressure of 10MPa while heated to 80° C., thereby obtaining an electrode layer sheetof 300 μm thickness.

(Formation of a Current Collector)

In a kneader, the following were kneaded: 100 parts of acetylene black,20 parts of a 10% solution of carboxymethylcellulose in water (Cellogen7H, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), 31.3 parts of acarboxyl-modified styrene/butadiene copolymer latex (an aqueous 40%dispersion, BM-400B, manufactured by ZEON CORPORATION), and 10.2 partsof soft water. Thereafter, the resultant was diluted with soft water toobtain an electroconductive adhesive agent having a solid concentrationof 30% (average particle diameter of the acetylene black, measured by alight scattering method: 0.5 μm). This electroconductive adhesive agentwas applied onto an aluminum foil of 30 μm thickness and then dried toobtain a current collector having an electroconductive adhesive agentlayer of 5 μm thickness.

(Production of Electrodes and an Electric Double Layer Capacitor)

A product obtained by cutting the current collector into a rectangle, 4cm×6 cm, was set onto the bottom of a mold, 4 cm×6 cm, so as to directthe face having the electroconductive adhesive agent layer upward, andthen 4.5 g of the above-mentioned powdery mixture was supplied into themold. The mixture was pressure-formed at a pressing pressure of 10 MPawhile it was heated at 80° C. In this way, electrodes wherein thethickness of their electrode layers was 300 μm were obtained.

Two out of the electrodes obtained as described above were used, and aseparator, 40 μm in thickness, made of cellulose fiber was sandwichedtherebetween to direct their electrode layers inwards. Furthermore, theresultant was sandwiched between two glass plates of 2 mm thickness, 5cm width and 7 cm height, so as to produce an element.

The element was heated under a reduced pressure at 200° C. for 3 hoursto remove impurities in the element. Thereafter, an electrolyticsolution was infiltrated thereinto under a reduced pressure. The elementwas put into a cylindrical container having a rectangular bottom andmade of polypropylene to produce an electric double layer capacitor. Theused electrolytic solution was a solution whereintriethylmonomethylammonium tetrafluoroborate was dissolved in propylenecarbonate at a concentration of 1.5 mol/L.

Example 2

An electrode layer, electrodes and an electric double layer capacitorwere produced in the same way as in Example 1 except that the followingwas used instead of the aqueous 40% dispersion of the carboxyl-modifiedstyrene/butadiene copolymer particles: an aqueous 40% dispersion of2-ethylhexyl acrylate/methacrylic acid/acrylonitrile/ethylene glycoldimethacrylate copolymer particles (particle diameter: 15 μm, and Tg:−50° C.).

Example 3

An electrode layer, electrodes and an electric double layer capacitorwere produced in the same way as in Example 1 except that: 170 parts ofactivated carbon were stirred with a Henschel mixer, 20 parts ofacetylene black were added thereto and mixed therewith over 10 minutes;and next 20 parts of an aqueous 40% dispersion of carboxyl-modifiedstyrene/butadiene copolymer particles were sprayed, added thereto, andmixed therewith over 10 minutes so as to obtain a powdery mixture, theparticle diameter of which was 144 μm.

Example 4

An electrode layer, electrodes and an electric double layer capacitorwere produced in the same way as in Example 1 except that: 170 parts ofactivated carbon and 20 parts of acetylene black were supplied into ahybridizer (manufactured by Nara Machinery Co., Ltd.) and subjected tomechanochemical treatment at a rotation number of 100 rpm over 1 minute;next, the resultant mixture was supplied into a fluidized bedgranulating machine (Agglomaster manufactured by HOSOKAWAMICRONCORPORATION); and 20 parts of an aqueous 40% dispersion ofcarboxyl-modified styrene/butadiene copolymer particles were sprayed,added thereto and mixed therewith, in flowing air, over 10 minutes so asto obtain a powdery mixture, the particle diameter of which was 32 μm.

The activated carbon, the acetylene black and the aqueous 40% dispersionof the carboxyl-modified styrene/butadiene copolymer particles that wereused in Examples 3 and 4 were the same as used in Example 1.

Comparative Example 1

To a mixture composed of 160 parts of activated carbon, 20 parts ofcarbon black, which were the same as used in Example 1, and 20 parts ofPTFE powder, were added 104 parts of ethanol, and the components weremixed. This mixture was pre-molded into a rectangular parallelepipedform, and was paste-extrusion-molded by use of a nozzle having anextrusion contraction ratio of 40 and a rectangular section. Theresultant extruded product was used, pressure-formed in the same way asin Example 1, and then dried at 250° C. for 30 minutes to removeethanol, thereby forming an electrode layer sheet of 300 μm thicknessand electrodes, the electrode layers of which each had a thickness of300 μm. The resultant electrodes were used to produce an electric doublelayer capacitor in the same way as in Example 1.

Comparative Example 2

160 parts of activated carbon and 20 parts of carbon black, which werethe same as used in Example 1, were incorporated and dispersed into asolution of a styrene butadiene rubber, which had no crosslinkedstructure and was produced by solution polymerization, in xylene, andthen the resultant mixture was dried to remove xylene. Thereafter, theresultant was pressure-formed in the same way as in Example 1, therebyforming an electrode layer sheet of 300 μm thickness and electrodes, theelectrode layers of which each had a thickness of 300 μm. The resultantelectrodes were used to produce an electric double layer capacitor inthe same way as in Example 1.

Comparative Example 3

The same aqueous dispersion of the carboxyl-modified styrene/butadienecopolymer particles as used in Example 1 was further diluted with waterto prepare an aqueous dispersion having a rubber particle concentrationof 1%. To 800 parts of this aqueous dispersion of the rubber particleswere added 170 parts of activated carbon and 20 parts of carbon black,which were the same as used in Example 1. These components were stirredand mixed. This mixture was dried to remove water content. The resultantaggregate was pulverized and granulated. The resultant powder wastreated by press-molding in the same way as in Example 1, therebyforming an electrode layer sheet of 300 μm thickness and electrodes, theelectrode layers of which each had a thickness of 300 μm. The resultantelectrodes were used to produce an electric double layer capacitor inthe same way as in Example 1.

Comparative Example 4

An electrode layer, electrodes and an electric double layer capacitorwere produced in the same way as in Example 1 except that 8 parts ofpolyethylene powder having a particle diameter of 20 μm were usedinstead of the aqueous dispersion of the carboxyl-modifiedstyrene/butadiene copolymer particles.

<Evaluation of the Electrode Layers, the Electrodes, and the ElectricDouble Layer Capacitors>

The electrode layers, the electrodes, and the electric double layercapacitors obtained in Examples 1 to 4 and Comparative Examples 1 to 4were evaluated about the following items. The results are shown in Table1.

(Tensile Strengths of the Electrode Layers)

These were measured in accordance with JIS K6251. Each of the electrodelayers molded into the sheet form was dried at 250° C. for 1 hour, andthen punched out into a form of a first-model, dumbbell-shaped testpiece. The test piece was subjected to a tensile test at an atmospheretemperature of 25° C. and a tensile speed of 20 mm/minute to measure themaximum load. This measure was repeated 3 times. The average of themaximum loads was divided by the sectional area of the sheet. Theresultant value was defined as the tensile strength of this electrodelayer. In order to measure the sheet tensile strength in the directionalong which the sheet was rolled, the punching-out was performed to makethe length direction of the dumbbell-shaped test piece consistent withthe rolling extrusion direction. As the tensile strength of theelectrode layer is larger, the layer is less cracked or broken and theshape-maintainability thereof is better.

(Evaluation Criterion)

-   ⊚: A result 20% or more better than that of Comparative Example 1    was obtained.-   ◯: A result better than that of Comparative Example 1 was obtained.-   Δ: A result equivalent to that of Comparative Example 1 was    obtained.-   X: A result poorer than that of Comparative Example 1 was obtained.    (Bending Strengths of the Electrodes)

Each of the resultant electrodes for electric double layer capacitorswas cut out into two rectangles of 100 mm length and 50 mm width as testpieces. A measurement was made in accordance with a method described inJIS K5600-5-1. As the device for the test, a device of type 1 was used.The diameters of used cylindrical mandrels in its bending region weretwo of 25 mm and 32 mm. Each of the test pieces was fitted onto the testdevice and a hinge was bent at an angle of 180° from a horizontal state.Thereafter, cracks in the electrode were observed with a loupe. The testpiece was evaluated based on the following criterion: ⊚

In the cases of the 25 mm and 32 mm diameters, no crack was observed: Inthe case of the 25 mm diameter, cracks were observed, but in the case ofthe 32 mm diameter, no crack was observed: ◯In the cases of the 25 mmand 32 mm diameters, cracks were observed: Δ

At the mandrel region(s), the test piece thereon fractured: X(Electrostatic capacity, and internal resistance)

At 25° C., each of the capacitors was charged up to 2.7 V from 0 V at aconstant current of 10 mA/cm² over 10 minutes, and then discharged up to0 V at a constant current of 1 mA/cm². The electrostatic capacitythereof was obtained from the resultant charging and discharging curve,and then divided by the weight of the electrode layer, which wasobtained by subtracting the weight of the current collector from theweight of the electrode, so as to give the electrostatic capacity perunit weight of the electrode layer. The internal resistance wascalculated from the charging and discharging curve by the calculatingmethod of the standard RC-2377 prescribed by Japan Electronics andInformation Technology Industries Association. TABLE 1 ElectrodeElectrostatic Internal layer Bending capacity resistance strengthstrength (F/g) (ΩF) Example 1 ◯ ◯ 53.3 5.6 Example 2 ⊚ ⊚ 55.3 5.6Example 3 ◯ ⊚ 55.7 5.5 Example 4 ◯ ⊚ 58.9 4.8 Comparative Δ ◯ 48.1 6.3Example 1 Comparative ◯ ◯ 48.3 6.2 Example 2 Comparative ◯ Δ 38.6 6.4Example 3 Comparative X X 35.4 7.7 Example 4

It can be understood from Table 1 that according to the invention(Examples 1 to 4), electric double layer capacitors having excellentelectrode layer strength, high capacity and small internal resistancewere obtained. Comparative Examples 1 to 4 were poorer, particularly, incapacity and internal resistance than the Examples.

The above has described the present invention in connection withembodiments which appear to be most preferable and most practical atpresent. However, the invention is not limited to the embodimentsdisclosed in the present specification, and can be appropriatelymodified within the scope which does not depart from the subject matteror the conception of the invention which can be understood from theclaims and the whole of the specification. It should be understood thatelectrodes for electric double layer capacitors, methods for producingthe same, and electric double layer capacitors with such modificationare also included in the technical scope of the invention.

INDUSTRIAL APPLICABILITY

As described above, according to the invention, it is possible toprovide a method for producing an electrode for giving an electricdouble layer capacitor having a high capacity, with a high productivity,through simplified steps.

1. A method for producing an electrode for an electric double layercapacitor, comprising: a step of mixing a particulate elastomer and acarbonaceous material with each other in a powdery form, therebyobtaining a powdery mixture; and a step of dry-forming the resultantpowdery mixture, thereby forming an electrode layer.
 2. The productionmethod according to claim 1, wherein the particulate elastomer is anelastomer having a crosslinked structure.
 3. The production methodaccording to claim 1, wherein the carbonaceous material comprisesactivated carbon as an active material.
 4. The production methodaccording to claim 3, wherein the carbonaceous material furthercomprises an electroconductivity additive.
 5. The production methodaccording to claim 4, which comprises a step of causing theelectroconductivity additive to adhere onto the surface of the activematerial by mechanochemical treatment.
 6. The production methodaccording to claim 1, wherein the powdery mixture is a mixture obtainedby fluidized bed granulation or fluidized bed multifunction modegranulation.
 7. The production method according to claim 1, wherein theparticle diameter of the powdery mixture is from 0.1 to 1000 μm.
 8. Theproduction method according to claim 1, wherein the dry-forming ispress-molding.
 9. The production method according to claim 8, whereinthe press-molding is performed inside a mold wherein a current collectoris set.
 10. The production method according to claim 1, wherein thepowdery mixture comprises, in 100 parts by weight thereof, 0.1 to 50parts by weight of the particulate elastomer and 50 to 99.9 parts byweight of the carbonaceous material.
 11. An electrode for an electricdouble layer capacitor, which is obtained by a production method asclaims in claim
 1. 12. An electric double layer capacitor, comprising anelectrode as claimed in claim 11.